Dina Ivina An ambulance can take twenty minutes to travel, which is five times longer than the amount of time the brain has after cardiac arrest. During this period, a person’s life often depends not on the doctor, but on the one who happens to be nearby. An automatic external defibrillator (AED) can use voice prompts to help a passerby perform resuscitation: it analyzes the heart rhythm itself and will not allow you to press a button if the shock is not needed.